Bug report

Bug description:

That leads to flaky behaviors.

This is my PYTHONSTARTUP file:

raise ModuleNotFoundError("oops!")

ModuleNotFoundError is just an example exception that can occur in PYTHONSTARTUP scripts.

This is how the regular REPLs handle it (both the basic REPL and the new REPL):

❯ PYTHONSTARTUP=t.py ./python
Python 3.15.0a1+ (heads/main:fbf0843e39e, Oct 18 2025, 11:24:43) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
Traceback (most recent call last):
  File "t.py", line 1, in <module>
    raise ModuleNotFoundError("oops!")
ModuleNotFoundError: oops!
>>> 

The traceback is shown and we proceed to the loop.

For the asyncio REPL, startup file exception always causes it to exit immediately -- we'd never want the REPL to exit because of a startup file error. On top of that, it doesn't report the exception properly, because it's racing with the interpreter shutdown at that point.

I've found 3 variants of the incorrect behavior:

1. The interactive daemon thread doesn't write the full traceback:

   ❯ PYTHONSTARTUP=t.py ./python -m asyncio
   asyncio REPL 3.15.0a1+ (heads/main:fbf0843e39e, Oct 18 2025, 11:24:43) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
   Use "await" directly instead of "asyncio.run()".
   Type "help", "copyright", "credits" or "license" for more information.
   Exception in thread Interactive thread:
   exiting asyncio REPL...
   Traceback (most recent call last):
   

2. The interactive daemon thread does flush, but after exiting is reported:

   ❯ PYTHONSTARTUP=t.py ./python -m asyncio
   asyncio REPL 3.15.0a1+ (heads/main:fbf0843e39e, Oct 18 2025, 11:24:43) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
   Use "await" directly instead of "asyncio.run()".
   Type "help", "copyright", "credits" or "license" for more information.
   Exception in thread Interactive thread:
   exiting asyncio REPL...
   Traceback (most recent call last):
     File "/home/bswck/Python/cpython/Lib/threading.py", line 1075, in _bootstrap_inner
       self._context.run(self.run)
       ~~~~~~~~~~~~~~~~~^^^^^^^^^^
     File "/home/bswck/Python/cpython/Lib/asyncio/__main__.py", line 104, in run
       exec(startup_code, console.locals)
       ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     File "t.py", line 1, in <module>
       raise ModuleNotFoundError("oops!")
   ModuleNotFoundError: oops!
   

3. The interactive daemon tries to acquire stderr at interpreter shutdown, and fails:

   ❯ PYTHONSTARTUP=t.py ./python -m asyncio
   asyncio REPL 3.15.0a1+ (heads/main:fbf0843e39e, Oct 18 2025, 11:24:43) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
   Use "await" directly instead of "asyncio.run()".
   Type "help", "copyright", "credits" or "license" for more information.
   Exception in thread Interactive thread:
   exiting asyncio REPL...
   Traceback (most recent call last):
     File "/home/bswck/Python/cpython/Lib/threading.py", line 1075, in _bootstrap_inner
       self._context.run(self.run)
       ~~~~~~~~~~~~~~~~~^^^^^^^^^^
   Fatal Python error: _enter_buffered_busy: could not acquire lock for <_io.BufferedWriter name='<stderr>'> at interpreter shutdown, possibly due to daemon threads
   Python runtime state: finalizing (tstate=0x00000000009c3818)
   
   Current thread 0x00007f6c4781cbc0 [python] (most recent call first):
     <no Python frame>
   Aborted (core dumped)
   

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-140287: Handle PYTHONSTARTUP script exceptions in the asyncio REPL #140288
• [3.14] gh-140287: Handle PYTHONSTARTUP script exceptions in the asyncio REPL (GH-140288) #148987
• [3.13] gh-140287: Handle PYTHONSTARTUP script exceptions in the asyncio REPL (GH-140288) #148988